Is this avoided by lifting the height of the hose as it enters from the hull? Is it fair to say that a regularly used engine will keep enough pumped out, but an unused engine without any height in the hose allows this?

P.S. I didn't allow this to happen, but I made it my problem now.

Mine rises to deck Hight from the exhaust at waterline , I did have a similar issue when the anti syphon valve packed it in . Filled the entire engine with water ,that's how I found out the stop cock didn't close properly . Just a thought .

Hi crewloose, do a google search "correct design for sea water cooled marine engine exhaust" and you'll pull up lots of diagrams that should help.

Here's one:
st2.ning.com/topology/rest/1.0/file/get/104608881?profile=original

It all depends on whether the cylinder head is above or below the waterline of the vessel. If it is below the waterline you will need the gooseneck exhaust section. Otherwise the air vent will do the job. On my boat I have a water flow indicator that comes off the vent and runs a trickle of water under the teak slats in the cockpit and drains into the cockpit drain. When I shut down, the water stops and it becomes the anti siphon valve. If you have no problem starting your diesel and it starts with minimal cranking you should not have a problem with salt water entering the exhaust valves. If the engine takes a lot of cranking to start, leave the salt water intake closed until the engine fires.